Experimental study of the propagation of ion acoustic solitons in a warm multi-ion plasma

The formation of ion acoustic solitons in an argon-helium plasma is studied as a function of the light-ion concentration. It is found that an initial perturbation does not break into solitons for $1\%\lesssim \alpha \lesssim 66\%$. This fact can be related to an increase in linear Landau damping and to nonlinear resonant particles, i.e., reflected ions in front of the potential hump and eventually trapped ions between two solitons. In the concentration range where solitons are formed (essentially for $\alpha \gtrsim 66\%$) the soliton's Mach number and width follow qualitatively the well-known theoretical dependence on amplitude. Detailed studies show, however, that the measured Mach number cannot be explained by fluid theories (namely the Korteweg-de Vries equation and Sakanaka's theory). Stationary solutions from theory which takes into account trapped electrons and reflected ions adequately describe the Mach number versus amplitude and light-ion concentration dependence.